Interpretive Summary: There is insufficient understanding of the nutritional physiology of pecan trees and orchards; thus, affecting nutmeat yield and quality, disease resistance and alternate bearing. An analysis of the rare-earth element composition of pecan and related hickory cousins found that they hyperaccumulate these elements and that these elements exhibit evidence of conferring biological advantages trees. There is evidence that rare-earth elements are beneficial nutrients for pecan; thus, opening up new aspects of nutrient management which potentially influences the profitability of orchard operations and use efficiency of orchard inputs.

Technical Abstract:
The nutritional physiology of pecan [Carya illinoinensis (Wangenh.) K. Koch] and other Carya species remains to be fully understood; thus, presenting a substantial knowledge gap in the horticulture of pecan and similar species. There is an especial lack of knowledge regarding the metabolic, ecophysiological, and horticultural relevance of rare-earth elements (REEs). The present study reports composition of the REE Metallomes component of the foliar ionomes of pecan and other North American Carya, and how accumulation of specific REEs relate to ploidy level and to accumulation of essential divalent micronutrients. REEs accumulation within the foliar ionomes of 12 Carya species growing on a common site and soil indicated that REEs were accumulated according to the Oddo-Harkins Rule, with Ce, La, Nd and Y (Ce>La>Nd>Y) being the dominant REEs accumulated, with accumulated concentration typically being La>Ce>Nd>Y> Gd>Pr>Sm>Dy> Er>Yb> Ho> Tb>Tm> Sc >Lu. Carya species quantitatively differed in their REE accumulation, with all but C. aquatica (Michx. f.) Nutt. accumulating REEs at much greater concentrations than non-Carya tree species, and with tetraploid Carya accumulating about twice the concentration of REEs as diploid Carya. C. tomentosa (Poir.) Nutt. was an especially heavy REEs accumulator at 890 ug.g-1 dw, and with C. aquatica being especially light at 89 ug.g-1. The accumulation of REEs was general for all species in that accumulation of any one REE was tightly linked (r = 0.94, and 0.81 for Ce) to any other REE. REE accumulation was negatively correlated with Ca accumulation, and positively correlated with Mn and Cu accumulation in diploid Carya; however, in tetraploid Carya accumulated Mg, Ca, and Fe was positively associated with REE concentration. Total concentration of REEs in pecan’s foliar ionome was 190 ug.g-1, approximately equivalent to the concentration of Mn. Circumstantial evidence is suggestive that one or more of the physiochemically similar REEs increases physiological plasticity and subsequent adaptive fitness to certain Carya spp., especially tetraploids; perhaps by substituting in certain metalloenzyme isoforms (e.g., for Ca2+, Mg2+, Fe2+/Fe3+, Mn2+ or Zn2+), intra-cellular cytosolic signaling, pigments where strong Lewis acids are required. The fact that all tetraploid Carya were relatively high REE accumulators and are also native to relatively xeric habitats, as compared to diploids which typically occupy mesic habitats, is suggestive that REEs play a major role in Carya speciation and adaptation to certain site limiting environmental stresses. It is proposed that REEs play an unknown metabolic role in pecan and most Carya spp., especially tetraploids, and are likely important components of The Biological Period Table for pecan and Carya, and merit further investigation regarding their roles in tree nutritional physiology.